HOW THE THERMOMETER WORKS

Bay of Plenty Beacon, Volume 8, Issue 24, 14 November 1944, Page 5

HOW THE THERMOMETER WORKS

PRINCIPLE DISCOVERED TWO

CENTURIES AGO

Everybody who has a thermometer pays it visits on cold days, to see what it has to say.

When it is very cold we find that the tiny column of mercury inside the glass tube stands round about the figure thirty-two marked on the scale beside the column. If it should become very hot in summer the mercury shoots up to ninety or there-

abouts

There is another kind of thermometer besides that, hanging on the wall smaller and thinner and made wholly of glass, with a mere thread of mercury inside. The doctor or the nurse puts it in our mouth to see how hot we are inside ? and if. the threads of mercury goes up to one hundred or more } nobody is pleased the*doctor least of all. How does this ingenious device, common to both instruments, work? It de-* pends on the fact that if a liquid is heated it and if cooled it shrinks. Water does so, except when, it becomes ice, and it always freezes or boils at the same temperature. Water does not make a good measurer because, of the behaviour at. the freezing point } when, becoming ice, it expands instead of contracting. But taking the freezing and boiling points of water as fixed r it was possible to make a, scale with 180 divisions, to tell the temperature in degrees between them.

Mercury like water ? expands and' contracts' but without -water's** peculiarity at the freezing pointy and is more easily handled. So } more than two centuries ago ; Fahrenheit made his first mercury with ISO degrees, between freezing and boiling, and it has the same. today ? with one alteration^ — Fahrenheit found that a mixture of ice and salt was thirty-two cokler than freezing wat©r t so he added them to his which now has 212 witlv the freezing point oi' water at thirty,two and the fopiling poiftt , at 212 degrees.